Adsorption and Diffusion Properties of Butanes in ZSM-5 Zeolite Membranes

Abstract

Adsorption isotherms and diffusion coefficients for n-C 4 and i-C 4 in the transport pathways through tubular ZSM-5 zeolite membranes were determined over a range of temperatures by a transient permeation method. The permeate response to step changes in the feed were measured, and the transport was modeled as Maxwell-Stefan diffusion with single site Langmuir adsorption in the zeolite. The heats of adsorption and the Langmuir parameters were comparable to values reported for MFI powders. The similarity of the membrane and powder isotherms indicates that butanes diffuse mainly through zeolite pores. Maxwell-Stefan diffusion coefficients for n-C 4 and i-C 4 in the ZSM-5 membranes were similar to those measured by other macroscopic techniques for zeolite crystals and membranes, and diffusion coefficients increased with increasing feed partial pressure for a high-quality membrane. The effective membrane thickness was also estimated from these transient measurements.

Modelling gas purification systems employing molecular sieve silica membranes

“…Thereafter, improvements in H 2 purity were minor and steady state started approaching after 100 min (6000 s). It is interesting to note that the evolution of concentrations as reported for ZSM-5 zeolite membranes complying with Langmuir adsorption [11] is similar to those produced by our model and verified by experimental tests. The time lag in a transient regime to reach steady state therefore depends on the initial gas concentration conditions, and the materials with dictates thermodynamic properties.…”

“…The ideal adsorbed solution (IAS) theory [10], together with the extended Langmuir model [5] was used to describe the binary permeation of gases through a silicalite-1 membrane. A transient model for the permeation of butanes (n-C 4 and i-C 4 ) based on Maxwell-Stefan diffusion with single site Langmuir adsorption in the ZSM-5 zeolite membranes [11].…”

Enhanced flux through double-sided zeolite membranes

“…The base case parameters (Table 1) were based on n-C 4 (kinetic diameter = 0.43 nm) in MFI (pore size ∼0.51-0.57 nm) on a stainless steel support at 273 K [5]. Fluxes for these parameters through F, FS, and FSF membranes for typical values of the adsorption equilibrium constant and support thickness are in Table 2.…”

“…Transport of pure components and mixtures in zeolites has been modeled by the generalized Maxwell-Stefan diffusion approach, with adsorption described by single-or multi-site Langmuir isotherms, with some success [4][5][6]. For diffusion of a single component through zeolites, the Maxwell-Stefan model takes into account the fact that the flux is driven by the chemical potential gradient rather than the partial pressure gradient across the membrane.…”

Integration of MFI Zeolite Membranes in the Light Gasoline Isomerisation Process

“…This has been previously observed with a mixture of hexane/3-methylpentane isomers: the diffusion coefficient of the linear compound (normal hexane) in silicalite (a MFI zeolite containing only silica) was equal to 4.5 × 10 -11 m 2 /s when 3-methylpentane was absent from the micro-pores, whereas it was lowered to 10 -11 m 2 /s when 83% of the micro-pore volume was filled with 3-methylpentane [12]. As the adsorption of branched paraffin isomers in MFI zeolite was decreasing with increasing temperature [13], the diffusion selectivity between the linear (faster) compounds and the branched compounds would therefore be more favoured at high temperatures. Unlike the situation observed at 2 bar, the concentration of 2,2 dimethylbutane in the permeate increased at 4 bar for temperatures above 300°C; it is possible that the transfer of dimethylbutane across the zeolite pores was favoured by higher adsorption of this compound in the zeolite due to its greater "flexibility" at high temperatures.…”